chore: add new keepalive_pool field to ApisixUpstream CR

[slayer321]

Type of change:

• [ ] Bugfix
• [ ] New feature provided
• [ ] Improve performance
• [ ] Backport patches
• [ ] Documentation
• [ ] Refactor
• [x] Chore
• [ ] CI/CD or Tests

What this PR does / why we need it:

Fixes #1883 adds the new field keepalive_pool for ApisixUpstream CR

Pre-submission checklist:

• [ ] Did you explain what problem does this PR solve? Or what new features have been added?
• [ ] Have you added corresponding test cases?
• [ ] Have you modified the corresponding document?
• [ ] Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[slayer321]

I have made all the required changes but I was not able to test it as I'm still stuck on the CRD's generation. One of the ways I tried doing it below

controller-gen crd paths=./pkg/kube/apisix/apis/... output:crd:dir=./samples/deploy/crd/v1

It is generating a whole new files for me. Please do let me know how can I generated the CRD's for this project.

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